In future optical networks, such techniques may be requested in which optical signals can be multiplexed in relay nodes and terminal station devices in optical networks, or in the midway point of transmission lines, for example. For one of promising techniques for multiplexing, an optical frequency division multiplexing technique is known. The optical frequency division multiplexing technique is a method in which information signals at different subcarrier frequencies are all-optically multiplexed on a carrier light beam at a single wavelength for transmission.
In the case where typical signal multiplexing techniques are sorted, there is an O/E/O method that implements signal multiplexing in which an optical signal (O) is temporarily converted into an electrical signal (E), the electrical signal is multiplexed, and the electrical signal is again converted into an optical signal (O). Moreover, there is an all-optical signal multiplexing method that does not involve optoelectric conversion into electrical signals.
The O/E/O method can adopt a multiplexing method such as time division multiplexing, phase multiplexing, and frequency multiplexing, for example. However, processing time becomes longer as the number of signals to be multiplexed is increased, and in addition to that, loads are gathered on devices on the downstream side as the number of major nodes is increased. Moreover, the use of electrical signal processing causes degraded energy efficiency, and puts limitations on processing speed. Under the present circumstances, a processing speed of a few tens GHz is the upper limit.
On the other hand, for the all-optical signal multiplexing method, a wavelength multiplexing method is known, for example. In the wavelength multiplexing method, a plurality of carrier light beams at different wavelengths is subjected to base band modulation, and multiplexed and demultiplexed using an optical multiplexing and demultiplexing filter. Therefore, in performing multiplexing at narrow wavelength (frequency) intervals, considerably highly accurate wavelength control may be necessary at a transmitting station and a wavelength demultiplexer because the multiplexing greatly depends on the stability of the oscillation wavelength of a light source. Thus, it is difficult to perform multiplexing at narrow wavelength (frequency) intervals.
Therefore, in these years, an optical frequency division multiplexing technique is known as the all-optical signal multiplexing method. In the optical frequency division multiplexing technique, signal multiplexing is performed using an all-optical modulator whose processing speed limit is far beyond the processing speed limit of the electrical signal. It is unnecessary to separate signals using a multiplexing and demultiplexing filter, and multiplexed signals can be electrically separated using a single O/E converter and a typical narrow band RF filter. Thus, the number of O/E converters to be allocated to multiplexed signals can be greatly reduced, and signals can be highly densely multiplexed and separated across a wide band.
In the optical frequency division multiplexing technique, in the case where optical frequency division multiplexing is performed in all-optical signal multiplexing, an optical modulator is used in the midway point of a transmission line or in the midway point of a node. The optical modulator for use in the optical frequency division multiplexing technique includes a lithium niobate optical modulator (a LNbO3 modulator) and an electroabsorption modulator, for example. However, these modulators are devices of a large insertion loss or of large polarization dependence. Moreover, since the optical modulator modulates electrical signals, the optical modulator has limitations on the processing speed.
Therefore, an all-optical modulator is known in which a nonlinear medium is used and cross phase modulation (XPM) between a carrier light beam and a signal light beam is used. In a method in which the cross phase modulation effect of a nonlinear medium is adopted, the processing speed is significantly faster than electrical processing because processing can be performed at a femto second order response speed due to the use of the nonlinear optical effect. Moreover, since the method uses an optical fiber as a nonlinear medium, the method is advantageous in that the insertion loss is small and coupling to a transmission line is excellent.    International Publication Pamphlet No. WO 2011/052075    Japanese Laid-open Patent Publication No. 2011-215603.
However, even the all-optical modulator using a nonlinear medium has polarization dependence. An ideal polarization state is a state in which the polarization state of a carrier light beam and the polarization state of a signal light beam are parallel with each other, and a phase shift amount caused by cross phase modulation becomes highest. On the other hand, the worst polarization state is a state in which the polarization state of the carrier light beam and the polarization state of the signal light beam are orthogonal to each other, and the maximum value of the phase shift amount caused by cross phase modulation is reduced to one-third, or reduced by 4.8 dB, for example, from the maximum value as compared with the ideal state. In the case where such an optical modulator is used to multiplex signals from multiple points, a level difference of 4.8 dB occurs between multiplexed signals at the maximum. In other words, it can be said that the all-optical modulator using cross phase modulation greatly depends on the polarization state of the signal light beam and the polarization state of the carrier light beam.
It is also considered to adopt a polarization diversity configuration for a method of solving polarization dependence. However, although this method can solve polarization dependence on one hand, the number of parts is increased to hike costs as well as to increase the insertion loss on the other hand.